Some ink jet print heads consist of stacks of steel or a mix of steel and polymer plates. The stack of plates form reservoirs and ink paths to route ink from a main reservoir to a nozzle plate. Typically, an actuator of some kind draws ink into a pressure chamber next to the nozzle plate and then pushes the ink out through the nozzles.
An example of an actuator consists of a piezoelectric actuator (PZT) that causes a diaphragm plate in the stack to flex and both pull ink into the reservoir and force it out through the nozzles. In some ink jet print heads, slabs of nickel plated PZT material are kerfed into individual actuators and epoxy-bonded to the diaphragm on the back side of the print head jet stacks. The epoxy used in production degrades at operating temperatures when exposed to air.
One possible approach uses tin-silver soldering but existing variants have too thick a bond line for many ink jet print heads. Typically, diffusion-soldered bonds are 50 to 100 micrometers thick. This thick of a bond degrades performance of the print head when inserted between a 50 micrometer PZT slice and a 20 micrometer diaphragm. Thinning will not work because the tin layer becomes too thin to break up and contain the natural oxide layer that forms on tin surfaces. The use of flux can cause problems because it either gets caught in the bond layer or may contaminate the print head.